1. Field of the Invention
The present application relates to a driving controller, and more particularly, a driving controller used for dynamically adjusting a voltage at a control terminal of a transistor so as to reduce conduction loss of a power switch.
2. Description of the Prior Art
In an electrical power conversion driving controller (e.g. a flyback circuit), a diode can be used as a rectification component. For example, a Schottky diode can be used because it has a lower forward voltage. However, power loss may be an issue. For example, when a voltage across a diode is 0.5 volts, and a root mean square of a current flowing through the diode is 10 amps, the power loss would be a product of the current and the voltage, which is 5 watts.
For reducing the power loss, a transistor may be used to replace a diode. For example, two terminals (e.g. a drain terminal and a source terminal) of a metal-oxide-semiconductor field-effect transistor (MOSFET) can be used to replace an anode and a cathode of a diode. After replacing the diode, if resistance of an enabled transistor is 10 mΩ, and a current flowing through the transistor is still 10 amps, the power loss would be a product of the resistance and the square of the current, which is 1 watt. The abovementioned values are merely examples. In this example, power loss may be reduced from 5 watts to 1 watt, so the power loss would be decreased using a transistor instead of a diode to be a rectification component in an electrical power conversion driving controller.
However, when the voltage across a transistor is converted to a forward voltage, the current may flow through a parasitic body diode within the transistor. A forward voltage across the body diode is larger than a forward voltage of the foresaid diode and is as high as 0.7 volts. This further increases the power loss. Hence, if the turning on and off of the transistor cannot be controlled properly, the power loss is increased further. Because of this, when the voltage across the transistor is close to 0 volts, the transistor can be turned off to reduce the conduction of the body diode so as to reduce the power loss. However, if the transistor is turned off too early, the transistor cannot properly reduce power loss.
For reducing the power loss, a transistor with a lower on-resistance can be intuitively selected, but it is difficult to improve the effect without adjusting the operation method. FIG. 1 illustrates a waveform diagram of a driving controller according to prior art. In FIG. 1, the operation current ID is a current flowing through a transistor. The operation VDS is a voltage across the transistor. The voltage Vdrv is a voltage at the control terminal of the transistor. The curve 181 corresponds to a transistor with a higher on-resistance, and the curve 182 corresponds to a transistor with a lower on-resistance. The voltage Voff is a threshold to be compared with the operation voltage VDS to accordingly turn off the transistor. As shown in FIG. 1, when using a transistor with a lower on-resistance, the curve of the operation voltage VDS will change from the curve 181 to the curve 182 corresponding to the same operation current ID. Hence, the time point of turning off the transistor can be changed from the time point t1 to the time point t2. Since the transistor is turned off earlier, the effect of reducing the power loss is worsened.
According to FIG. 1, when turning off the transistor, the voltage Vdrv at the control terminal of the transistor is pulled down from a maximum voltage, so it is difficult to shorten the turn-off delay time, and the operation may slow down the turning off of the circuit.